Reddit user HeySoloSoyYo posted a thread showing how they brought Carl Johnson, also known as CJ in 2004's Grand Theft Auto: San Andreas, to life inside Red Dead Redemption 2 's online multiplayer. One player changed that in detailed fashion by bringing one of San Andreas' most notorious gangsters to life in Red Dead Redemption 2.
However, characters jumping franchises is not something Rockstar has played around with, instead keeping all the franchises fairly isolated from each other. Returning characters is nothing new for Rockstar Games, with characters like John Marston and Brucie Kibbutz making repeat appearances in the Red Dead Redemption and Grand Theft Auto series, respectively.